‘Tell the Doctor, winner takes all.’ It opens on a shot of chess pieces, setting up the wider conflict between the Guardians that, after eight episodes of a cackling Valentine Dyall, begins to coalesce in this episode. The TARDIS control room has never looked better than it does here, bathed in warm, amber light. It reminds me, slightly, of the 13th Doctor’s TARDIS without the slightly obscene thrusting crystal. And right the way through the episode, the lighting is great – dark corners below decks on the yacht, low lighting in the corridors and dining room. It’s a different perspective given some other directors of the period blamed production shortcoming on the fact it was impossible to get the technicians to turn the lights down.

While this period’s Hartnell influences generally amount to random continuity discussions at the top of each episode, and loads of companions, Barbara Clegg’s script genuinely feels like it’s resurrecting the ethos of things like The Sensorites or The Space Musuem. This episode is slow burn, but nicely mysterious. The powered-down TARDIS and the White Guardian’s dire warnings of danger establish a vague sense of unease from the outset, which is carried though in both Marriner’s creepy ability to apparently look into the Ship from the outside, and the crew’s collective amnesia about the circumstances of coming aboard. Unlike Carnival of Monsters, which has a vaguely similar setting, this plays its cards closer to its chest: right up until the cliffhanger it’s more than likely that this is an Edwardian yacht – not, as it turns out, a spaceship that just happens to look like one.

It’s all much more subtle than anything else in the season so far. And it puts the regulars in interesting places: Turlough gets to socialise below decks, seemingly adopted as an honorary crewmember as he gets a hat plonked companionably on his head. Tegan, reluctantly, is wooed by Marriner, getting to see how the other half live in relative oak-panelled luxury. The Doctor flips between the two worlds, starting to piece together a sense of something very wrong. I’m really enjoying this.

Next episode: Enlightenment – Part Two